2x sim phone - problems making/receiving calls on smarty 4g-volte

[BlueSmarties]

Dual sim phone with smarty in slot one and EE in slot 2.
Priority for calls, sms, and data is set to sim2 = EE network.
never had this many problems before the switch to 4g-volte but anecdotally it feels like it's getting worse.

Full signal on smarty and EE and data connected.
Trying to make or receive calls on smarty sometimes gets through and sometimes does not. Mostly does not.
When I make smarty the priority for data then it seems to work better it's hard to do empirical testing.

What doesn't help is that when dialling a number there will be a dial tone but the call never collects, falsely leading you to think they didn't answer. However if you don contact the other caller, they will say that they could hear me saying "hello" and they did too. I hear nothing but a dial tone.
Again hard to test authentically.

Everything works fine with wifi calling, both sims are going over wifi separately. - I note some newer phones allow a second sim to pretend inside the phone that another sim's data can be used for pseudo wifi calling. Sadly not mine.

So questions.
1) Does a smarty sim need the smarty data connection being active to work over 4g-volte for calling?
2) Any link to a faq where this is written would be great.

[Linsey]

Hey @BlueSmarties 

A Smarty SIM does not need an active data connection for 4G VoLTE calling to work.

VoLTE (Voice over LTE) operates separately from mobile data.

As long as VoLTE is enabled on your phone and supported by SMARTY, you can make 4G calls even with mobile data turned off.

Just keep in mind that your phone needs to support VoLTE and be compatible with SMARTY's VoLTE service.

Checking if your device is 4G-VoLTE compatible | SMARTY Help Centre

For more info regarding 4G VoLTE (4G Calling) please refer to our help article: 

Switching off 3G | SMARTY Help Centre

[BlueSmarties]

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A Smarty SIM does not need an active data connection for 4G VoLTE calling to work.
VoLTE (Voice over LTE) operates separately from mobile data.
As long as VoLTE is enabled on your phone and supported by SMARTY, you can make 4G calls even with mobile data turned off.

I appreciate how you may have interpreted my post to be discussing how mobile data usage might affect the calling. Let me rephrase my question::
Does the smarty sim not having data enabled at all and the priority being for data connection to be using the second sim affect volte calling?
In my experience and testing, the answer is yes.

As I said above from my testing, there is a significant problem when the data package in use is using the EE sim not the smarty sim. When I change data use to be smarty, this problem appears to go away. Coincidence not.

Would you care to address the specifics of secondary sim being used for the data connection and how they may affect the smarty sim being able to use volte?
I see from googling that this appears to be a problem experienced by others on other networks leading me to think it's a limitation of volte - along with my experiments.

Just keep in mind that your phone needs to support VoLTE and be compatible with SMARTY's VoLTE service.

Let's just put a cork in that one. My phone is compatible and it's correctly setup with VoLTE enabled. Which by the way is working fine with the EE sim. The smarty list isn't well maintained.

[BlueSmarties]

I should add. My wife who is in the process of moving over to smarty (and may not do so) sees a similar problem in her phone.
The smarty sim refuses to make or take calls correctly if the data priority is set to her secondary sim also with another provider MVNO not 3.
Disable that sim for data use and smarty works correctly.
Her phone also on the compatible list.

So are there issues with dual sims?